1. Field of the Invention
This invention relates to a heavy duty pneumatic radial tire having an excellent belt durability.
2. Description of Related Art
As the conventional heavy duty pneumatic radial tire, there is known, for example, a tire comprising a carcass toroidally extending between a pair of bead portions and containing cords embedded substantially in a radial direction, a belt disposed on an outside of the carcass in the radial direction and a tread disposed on an outside of the belt in the radial direction, wherein the belt is comprised of four or more belt layers, cords of adjacent two belt layers among these layers are crossed with each other with respect to an equatorial plane of the tire.
Since such a tire is run under heavy load, the belt is subjected to a large bending deformation in a circumferential direction so as to be made flat at a ground contact region. Moreover, the cords of the adjacent two belt layers are embedded in these layers so as to be inclined in opposite directions with respect to the equatorial plane, so that shearing strain is created between both the belt layers due to pantograph deformation of cords in these belt layers.
Such a shearing strain becomes larger as the width of the belt layer becomes wider and as it approaches to an end of the belt layer. That is, a largest shearing strain is created between an end of a widest-width belt layer and an end of a wider-width belt layer in two belt layers adjacent to the widest-width belt layer, i.e. between ends of main cross belt, whereby crack is first created between the ends of the widest-width belt layer and the wider-width belt layer and finally progressed into separation failure between the belt layers.
Moreover, it is said that in order to control such a separation failure between belt layers, it is effective to reduce the aforementioned shearing strain between the belt layers by making large the inclination cord angle of all belt layers inclusive of the main cross belt with respect to the equatorial plane of the tire.
However, it has been confirmed that it is impossible to sufficiently control the separation failure between belt layers even when the inclination cord angle is made large in all of the belt layers.
The inventor has examined and studied the separation failure between belt layers in the heavy duty pneumatic radial tire in detail and confirmed that in case of running the heavy duty pneumatic radial tire on rough road, bad road and the like scattered with rocks, ground stones and so on, when the belt is comprised of three or more belt layers inclusive of the main cross belt and a neighboring narrower-width belt layer is disposed on the outside of the belt in the radial direction and adjacent thereto, separation failure is created between end portions of the main cross belt and the neighboring narrower-width belt layer and largely affects the belt durability.
Now, the inventor has made various studies with respect to the mechanism of creating such a separation failure and found out that when the tire rides on projections such as rocks, ground stones and the like, the tread deforms so as to envelop the projection, during which the neighboring narrower-width belt layer near to the tread is mostly affected by the above deformation, while since the three or more belt layers (inclusive of the main cross belt) arranged at the inside of the neighboring narrower-width belt layer in the radial direction are high in the bending rigidity, the deformation concentrates only in the neighboring narrower-width belt layer arranged at the outside of the main cross belt in the radial direction and hence a large shearing strain is created between the main cross belt and the neighboring narrower-width belt layer to finally bring about the occurrence of separation failure between belt layers.
The invention is based on the above discovery and lies in a heavy duty pneumatic radial tire comprising a carcass toroidally extending between a pair of bead portions and containing cords embedded substantially in a radial direction, a belt disposed on an outside of the carcass in the radial direction and a tread disposed on an outside of the belt in the radial direction, wherein the belt is comprised of four or more belt layers, cords of adjacent two belt layers among these layers are crossed with each other with respect to an equatorial plane of the tire, and when a main cross belt is constituted with a widest-width belt layer and a wider-width belt layer in two belt layers adjacent to the widest-width belt layer, an inclination angle R of cords in a neighboring narrower-width belt layer arranged at an outside of the main cross belt in the radial direction and adjacent thereto with respect to an equatorial plane of the tire is made larger than an inclination angle H of cords in a belt layer arranged outward in the radial direction among the widest-width and wider-width belt layers.
When the inclination angle R is made larger than the inclination angle H as mentioned above, the change in the inclination angle of the cord of the neighboring narrower-width belt layer in the riding on the projection is decreased, whereby the shearing strain produced between the neighboring narrower-width belt layer and the main cross belt is decreased to effectively prevent the separation failure between belt layers.
Particularly, when the inclination angle R is made larger by 3-35xc2x0 than the inclination angle H, the shearing strain is surely decreased to strongly control the occurrence of the separation failure between belt layers.
When the inclination angle R is within a range of 20-40xc2x0 and the inclination angle H is within a range of 5-35xc2x0, the occurrence of the separation failure between belt layers can be more strongly controlled while ensuing the hoop effect.
Furthermore, it is favorable that a belt protection layer of cords having an elongation at break larger than that of the cord in the belt layer is disposed between the belt and the tread, whereby an external injury from the outer surface of the tread into the belt can be prevented.
And also, it is favorable that an inclination angle N of cords in an innermost belt layer arranged inward in the radial direction is made within a range of 5-15xc2x0 with respect to the equatorial plane of the tire, whereby the growth of tire size and size growth during the running can effectively be decreased.
Further, it is favorable that a width M of the innermost belt layer is within a range of 0.33-0.70 times a maximum width of the main cross belt. In this case, the occurrence of separation failure from the end of the belt layer can be suppressed.
Moreover, it is favorable that a width Q of the widest-width belt layer is within a range of 0.60-0.85 times a tread width T. In this case, sufficient hoop effect can be developed while effectively suppressing the separation failure from the end of the belt layer.